July 1952 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
Many moons ago while in the USAF, I designed
and built a pair of speaker enclosures out of pine wood in the base woodshop. Each
had a separate bass, midrange, and tweeter speaker (and the acoustically
transparent cloth for the front) - all bought at the downtown
Radio Shack. An issue of
Popular Mechanics magazine had guidelines for the cabinet layout and made
passing mention of the need to install an adequate frequency crossover unit in order
to obtain the best performance. That article did not contain any information for
designing your own crossover network, so I went about trying to figure out my own,
based on a fairly limited knowledge of circuit theory and how to match impedances.
There was no Internet back then for conveniently looking up that sort of stuff.
I came up with a circuit that managed to work, but I honestly have no idea whether
the frequency division was anywhere near what I thought it should be. I probably
would have done just as well (or better) to have bought a high quality 3-speaker
unit sold for car hi-fi stereos, and mounted them in the cabinets. See also "Dividing
Networks" in the December 1949 issue of Radio & Television News.
Loudspeaker Crossover Design
Greater realism through correct dual-speaker phasing - A direct
approach to Loudspeaker Crossover Design
By Norman H. Crowhurst
Some points about the functioning of loudspeaker crossover networks should be
clarified. Most classical treatments derive crossover networks from wave filter
theory, which in turn is derived from the theory of artificial lines. The resulting
designs may not satisfy all the requirements of a crossover network in the best
The first and most obvious requirement of a crossover network is to deliver low-frequency
energy to one speaker and high-frequency energy to another. This is usually taken
care of reasonably well, using a roll-off slope to suit the designer's whim.
A second factor (and one that often receives less attention) is the impedance
presented to the amplifier by the combined network. Frequently this varies widely
over the audio range and includes sizeable reactive components in the vicinity of
the crossover frequency. To obtain best performance from the amplifier, a constant,
resistive impedance should be presented to it as a load.
The third requirement often receives even less attention. This is the realism
of the acoustic output from the combination. While this is in some respects a matter
of individual conditioning and preference, it depends fundamentally on certain electro-acoustic
requirements. One of the most important, and frequently the one least considered,
is the phasing, or apparent source of the sound output.
Fig. 1 - The apparent source of sound shifts when speakers are
not in phase.
Fig. 2 - Examples of crossover networks for two-way loudspeaker
Chart for finding exact values of inductance and capacitance
for any of the networks shown in Fig. 2. Use of the chart for a typical solution
(Fig. 2-e) is illustrated in Fig. 3 on the opposite page.
Fig. 3 - How the chart on the opposite page [above] is used to
find inductance and capacitance values for Fig. 2-e. Each speaker has a 40-ohm
Fig. 4 - Crossover network component values for the circuit of
Fig. 2-e derived by the method shown in Fig. 3.
Tests have shown that phase distortion with single speakers is not normally detectable,
but when two sources are employed, the phase relations between them influence the
character of the radiated sound field. The frequency response as registered by a
good pressure microphone may be flat, but what about the wave-shapes? The pressure
microphone does not answer that, but a pair of human ears can detect such phase
peculiarities, and failure to consider this factor has made many dual-unit combinations
sound noticeably unreal, even though their frequency response may look perfect,
and there may be no measurable distortion.
Source of the Sound
The reader may have checked two speakers for phasing by listening to them while
connections to one of them are reversed. Standing some distance in front of them,
on the center line (as in Fig. 1), when they are correctly phased the sound seems
to come from a point midway between the two units; but when incorrectly phased,
two effects can be noticed: There is a deficiency of low frequencies (due to cancellation
effects) and the source of sound at higher frequencies no longer seems to be associated
with the units actually radiating it. This is because the air-particle movement
caused by the radiated sound is no longer back and forth along a line from the common
source, but approximately at right angles to it. The sound field around the listener's
head is perpendicular to what it should be, causing, through our binaural perception,
the confused impression which may be called "dissociation effect."
A similar dissociation effect will occur with dual units driven by a crossover
system, at any frequency where the two speakers are out of phase. It is best to
keep the two units close together. Some favor putting the smaller unit on the axis
of the larger one and immediately in front of it. But, however the units are arranged,
the dissociation effect can become noticeable if there is an out-of-phase condition
at some frequency near the crossover point. The relative phase at crossover can
be adjusted by positioning the diaphragms on their common axis so the wave from
the low-frequency unit emerges in phase with that from the high-frequency one. When
the units are mounted side by side on the same baffle, the sound should emerge in
phase at the baffle surface.
But how does the crossover network affect the relative phase at frequencies near
crossover? This question often seems to be overlooked, and neglecting it can cause
the trouble just described. Some networks, of the kind employing two or more reactances
for each unit, have values adjusted to give an accentuated frequency rolloff. For
example, the networks shown at Fig. 2, c and d, using values given by the chart
in this article, are of the constant-resistance type, giving a rolloff of 12 db
per octave; the phase difference between the outputs is always 180 degrees; but
using values designed for a sharper rolloff, the phase difference is not the same.
At frequencies near crossover, phase difference changes rapidly. Some out-of-phase
effect in the vicinity of the crossover frequency is unavoidable unless a constant-resistance
type network is used. It is fortunate that this type takes care of both the second
and third requirements mentioned above.
The chart may be used to design any of the six types of crossover network illustrated
in Fig. 2. Those at a and b give a rolloff of 6 db per octave and a constant phase
difference of 90 degrees. For best results the positions of the two diaphragms should
be adjusted so the difference in their distances from the face of the baffle is
about one-quarter wavelength at the crossover frequency. The phase difference will
not be serious within the range where appreciable energy is coming from both units.
The networks at c and d give a rolloff of 12 db per octave, and a constant phase
difference of 180 degrees, which means that reversing connections to one unit will
bring the phase right. The units should be mounted so their diaphragms are in the
For cases where the frequency response of the units used requires a roll-off
steeper than 12 db per octave, the networks shown at e or f are recommended. These
give a rolloff of 18 db per octave, and a constant phase difference of 270 degrees.
This means that mounting the diaphragms a quarter-wavelength apart at the crossover
frequency will give in-phase outputs by appropriate connection.
All these networks are designed to present a constant, resistive impedance to
the amplifier over the entire frequency range.
The Impedance Varies
One more point is often overlooked: The networks are designed on the theory that
they are feeding resistance loads of the same value as the nominal voice-coil impedance.
The voice-coil impedance is not pure resistance, so the performance of the networks
is altered. The most serious effect is usually due to the inductance of the low-frequency
unit voice coil. By using networks a, d, or e, each of which feeds the low-frequency
unit through a series inductance, this effect can be overcome by subtracting the
voice-coil inductance value from the network inductance value derived from the chart.
Even if the available data is insufficient to allow this, these networks will minimize
the effect, because the inductance of the voice coil will add very little to the
effective inductance of the network. In the other networks the shunt capacitor combined
with the voice coil inductance will cause a greater variation in input impedance.
Each diagram in Fig. 2 has the inductors and capacitors marked with symbols.
These identify the reference line (in the bottom part of the design chart) to be
used for finding each component's value. Fig. 3 illustrates the use of the chart
to find values for a network of the type of Fig. 2-e, and Fig. 4 shows the actual
circuit calculated in this way for a crossover frequency of 500 cycles, at 40 ohms
The input impedance is the same as each speaker voice-coil impedance. Some prefer
to design the crossover network for 500-ohms impedance and use matching transformers
at the outputs to feed the individual voice coils. This method has two advantages:
The two units need not have the same voice-coil impedance; and smaller capacitors
can be used. The range of impedances covered by the chart extends up to 500 ohms
to include such designs.
One modern trend has been to use separate amplifier channels for each unit. In
this case the chart can be used for designing an inter stage filter to separate
the channels, by making the following adjustments: multiply all impedance values
by 1,000 (this means the impedance used to terminate each output as a grid shunt);
change inductance values to henries instead of millihenries; divide capacitor values
by 1,000. Suitable networks for this application are a, c or e, since these allow
the input to each amplifier circuit to be grounded on one side.
The characteristics of the final out-put from any type of network and combination
of speakers can only be predicted accurately if the source (amplifier) impedance
is known. This can be measured with simple equipment by the methods described in
the article "Audio Impedance Measurements," by James A. Mitchell, in the April,
1952, issue of Radio-Electronics.
The amplifier impedance should be as low as possible, and practically constant
over the entire frequency range to be reproduced. With present-day components, this
can be achieved only by using multiple low-impedance output triodes, or by carefully-designed
inverse feedback circuits. The series of articles "Audio Feedback Design," by George
Fletcher Cooper (Radio-Electronics, October, 1950 - November, 1951), covers many
applications of feedback to amplifier circuits.
Posted April 11, 2022